Friction stir processing was applied to a magnesium alloy to generate various grain sizes with the same intense basal texture. Subsequent tensile deformation along two orthogonal directions by easy activation or inhibition of basal slip follows the Hall-Petch relationship between yield stress and grain size in both directions. The Hall-Petch parameters are texture dependent. Easy activation of basal slip introduces lower values, while inhibition of basal slip leads to higher values. The Hall-Petch parameters are interpreted by texture and microstructural factors. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.